Nuclear Physics Level 2

Comments (0)

Transcript of Nuclear Physics Level 2

StabilityNuclear ReactionsRadio-isotope DatingHistoryRoentgenCurieAlphaBetaGammaRadiation DetectorsGeiger-Muller TubeCloud & Bubble ChamberBasicsAlpha DecayBeta DecayPositron Decay [beta + decay]Gamma DecayBecquerelDecay SeriesHalf-LifeAntoine Henri Becquerel (15 December 1852 – 25 August 1908) was a French physicist, Nobel laureate, and one of the discoverers of radioactivity. He won the 1903 Nobel Prize in Physics for discovering radioactivity.In 1896, while investigating phosphorescence in uranium salts, Becquerel accidentally discovered radioactivity. Investigating the work of Wilhelm Conrad Röntgen, Becquerel wrapped a fluorescent substance, potassium uranyl sulfate, in photographic plates and black material in preparation for an experiment requiring bright sunlight. However, prior to actually performing the experiment, Becquerel found that the photographic plates were fully exposed. This discovery led Becquerel to investigate the spontaneous emission of nuclear radiation.

Describing his method to the French Academy of Sciences on 24 January 1896, he said:"One wraps a Lumière photographic plate with a bromide emulsion in two sheets of very thick black paper, such that the plate does not become clouded upon being exposed to the sun for a day. One places on the sheet of paper, on the outside, a slab of the phosphorescent substance, and one exposes the whole to the sun for several hours. When one then develops the photographic plate, one recognizes that the silhouette of the phosphorescent substance appears in black on the negative. If one places between the phosphorescent substance and the paper a piece of money or a metal screen pierced with a cut-out design, one sees the image of these objects appear on the negative. … One must conclude from these experiments that the phosphorescent substance in question emits rays which pass through the opaque paper and reduces silver salts"Wilhelm Conrad Röntgen (27 March 1845 – 10 February 1923) was a German physicist, who, on 8 November 1895, produced and detected electromagnetic radiation in a wavelength range today known as x-rays or Röntgen rays, an achievement that earned him the first Nobel Prize in Physics in 1901.During 1895 Röntgen was investigating the external effects from the various types of vacuum tube equipment when an electrical discharge is passed through them. In early November he was repeating an experiment with one of the tubes in which a thin aluminium window had been added to permit the cathode rays to exit the tube but a cardboard covering was added to protect the aluminium from damage by the strong electrostatic field that is necessary to produce the cathode rays. He knew the cardboard covering prevented light from escaping, yet Röntgen observed that the invisible cathode rays caused a fluorescent effect on a small cardboard screen painted with barium platinocyanide when it was placed close to the aluminium window.Röntgen speculated that a new kind of ray might be responsible. 8 November was a Friday, so he took advantage of the weekend to repeat his experiments and make his first notes. In the following weeks he ate and slept in his laboratory as he investigated many properties of the new rays he temporarily termed X-rays, using the mathematical designation for something unknown. Although the new rays would eventually come to bear his name in many languages where they became known as Röntgen Rays, he always preferred the term X-rays.Marie Skłodowska Curie (November 7, 1867 – July 4, 1934) was a physicist and chemist of Polish upbringing and, subsequently, French citizenship. She was a pioneer in the field of radioactivity, the first person honored with two Nobel Prizes,[1] and the first female professor at the University of Paris.Pierre Curie (15 May 1859 – 19 April 1906) was a French physicist, a pioneer in crystallography, magnetism, piezoelectricity and radioactivity, and Nobel laureate. In 1903 he received the Nobel Prize in Physics with his wife, Maria Skłodowska-Curie, and Henri Becquerel, "in recognition of the extraordinary services they have rendered by their joint researches on the radiation phenomena discovered by Professor Henri Becquerel."PierreMariePierre Curie studied ferromagnetism, paramagnetism, and diamagnetism for his doctoral thesis, and discovered the effect of temperature on paramagnetism which is now known as Curie's law. The material constant in Curie's law is known as the Curie constant. He also discovered that ferromagnetic substances exhibited a critical temperature transition, above which the substances lost their ferromagnetic behavior. This is now known as the Curie point.Pierre worked with his wife Marie Curie in isolating polonium and radium. They were the first to use the term "radioactivity," and were pioneers in its study. Their work, including Marie's celebrated doctoral work, made use of a sensitive piezoelectric electrometer constructed by Pierre and his brother Jacques.Pierre and one of his students made the first discovery of nuclear energy, by identifying the continuous emission of heat from radium particles. He also investigated the radiation emissions of radioactive substances, and through the use of magnetic fields was able to show that some of the emissions were positively charged, some were negative and some were neutral. These correspond to alpha, beta and gamma radiation.Her achievements include the creation of a theory of radioactivity (a term coined by her), techniques for isolating radioactive isotopes, and the discovery of two new elements, polonium and radium. It was also under her personal direction that the world's first studies were conducted into the treatment of neoplasms (cancers), using radioactive isotopes.

While an actively loyal French citizen, she never lost her sense of Polish identity. She named the first new chemical element that she discovered (1898) polonium for her native country, and in 1932 she founded a Radium InstituteWhat is it?IonisingabilityPenetrationWhat is it?IonisingabilityPenetrationWhat is it?IonisingabilityPenetrationA helium nucleus that has been ejected from large radioactive atom.Released at about 5% of the speed of light [c].Highly ionising due to it's relatively large mass [approx 4u]Because alpha particles have such low penetrating force, they are stopped by human skin, presenting little danger unless the source is swallowed. This was the sad fate of Russian ex-spy Alexander Litvinenko, thought to be the first person to die from acute radiation poisoning as a result of ingesting the alpha emitter polonium. Other known alpha emitters include americium (found in smoke detectors), radium, radon gas, and uranium. When coupled together with certain other radioactive substances, alpha emitters can agitate neutron emitters to release the neutrons. Neutron emission is a critical part of nuclear reactor and nuclear weapons design.More FactsDue to its ionising ability an alpha particle will not travel far evenn through air, perhaps a few cm.A sheet of paper or some skin will stop it.A high speed electron emitted from the nucleus.Release somewhere between 50% and 90% of the speed of light [c]Moderately ionising.Although they travel much faster than alpha particles they have a much smaller mass. [approx 1/8000th]Beta particles can travel perhaps a metre through air or can be stopped with a few mm of aluminiumLike other radioactive substances, beta particle emitters are used in radioisotope thermoelectric generators, used to power space probes, not to mention remote Russian lighthouses. These lighthouses are actually a significant environmental concern, as they contain more strontium than what was released in the Chernobyl fire.MorefactsAn electromagnatic photon, not a particle. Similar to light but with much more energy.It takes energy away from energetic nuclii [in effect cooling them down]Gamma rays are very weakly ionisingAs they do not readily ionise they travel great distances.They can be stopped by many cm of lead [high density].Power drops as with the Inverse Square LawThe powerful nature of gamma-rays have made them useful in the sterilising of medical equipment by killing bacteria. They are also used to kill bacteria in foodstuffs to keep them fresher for longer.

In spite of their cancer-causing properties, gamma rays are also used to treat some cancerous growths. Multiple concentrated beams of gamma rays are directed on the growth in order to kill the cancerous cells. The beams are aimed from different angles to focus the radiation on the growth while minimising damage to the surrounding tissues.Other factsradiation240-100All of the elements up to lead [Pb, Z = 82]have at least one stable isotope.The remaining naturally occuring elements up to uranium [U, Z = 92] are ALL unstable and therefore radioactive.All man-made elements from plutonium [Pu, Z = 93] are ALL unstable with very short half-lives. Which is why they are not found in nature.The neutrons in a nucleus 'space out' the protons so that the coulomb repulsion force doesn't split it apart.Light elements have a roughly 1:1 ratio of protons to neutrons, heavier elements need relatively more neutrons.The heaviest man-made element is the newly named Copernicium [Cp, Z = 112].There may exist an 'Island of stability' at or around element 120StableUnstableTwo rules apply during nuclear reactions.The total nucleon number is conservedThe total proton number is conservedAlpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (two protons and two neutrons bound together into a particle identical to a helium nucleus) and transforms (or 'decays') into an atom with a mass number 4 less and atomic number 2 less.Alpha decay occurs when:The nucleus is too big [massive]Lightest element to undergo alpha decay is tellurium [Z = 52]Beta decay is a type of radioactive decay in which an atomic nucleus emits an electron and transforms (or 'decays') into an atom with an identical mass number and atomic number 1 more.Beta decay occurs when:The nucleus has too many neutrons compared to protonsPositron [beta positive] decay is a type of radioactive decay in which an atomic nucleus emits an electron and transforms (or 'decays') into an atom with an identical mass number and atomic number 1 less.Positron decay occurs when:The nucleus has too many protons compared to neutronsA positron is an ANTIMATTER particleIn gamma decay a nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons). The number of protons (and neutrons) in the nucleus does not change in this processGamma decay occurs when:The nucleus has too much energy to remain stableHalf-LifeActivityEquationsThe half life of a radioactive isotope is stated as:The time it takes for the number of original radioactive nuclei to fall to one half.OrThe time it takes for the activity of a radioactive source to fall to one half of its original value.It gives a characteristic exponential decay curve.The activity of a radioactive source is given in disintegrations per second.Or, bequerels [Bq]It is linked to the number of radioactive nuclei by the following equation proportionality.Question 1A radioactive source decays from 120Bq to 15Bq in 13.5 minutes.

How long is one half life?Question 2A radioactive source with a half life of 13ms is left for 1 second. There were originally 13,500,000,000 nuclei in the sample

How many nuclei are present after 1 second?How many original nuclei are present after 1 second!Question 3A radioactive sample contains 12 million nuclei and the activity is 3500Bq.

How long is one half life?Question 4A radioactive source with a half life of 34.7 years decays from 45000Bq to 12Bq.How long does this take?Question 5A radioactive source of half life 13.6s has a measured activity of 8900Bq.

How many atoms are in the sample?The constant of proportionality is called the disintegration constant.The equation now looks like:Exponential decayIf the number of original nuclei [or activity] is plotted against time we get an exponential decay curve.This curve can be described mathematically by:Or for activity...QuantitiesOriginal number of atoms

Number of atoms at time t

Original Activity [Bq]

Activity at time t [Bq]

Disintegration constant

Elapsed time

Half-life

Numbr of half livesCarbonPotassium-ArgonThe first method was invented by Williard Libby in the 1950s. He discovered that radiocarbon, C-14, had a half life of 5730 years.Moreover, although the C-14 on the surface of the Earth is constantly decaying away, it is also being produced. Cosmic rays (high energy protons from space) hit the atmosphere and produce neutrons; these neutrons are absorbed by a nitrogen atom in the atmosphere, which then immediately emits a proton, creating a new C-14. The net effect is that the C-14 is produced at the same rate as it decays, so the level of C-14 stays constant. Its level is is one part in a trillion, i.e. 10^-12 of ordinary carbon.Plants absorb this carbon when they breathe in carbon-dioxide. So the carbon in plants consists of one part in a trillion C-14. We eat plants (and animals that eat plants, and animals that eat other animals) and the result is that the carbon in our bodies is also one part in a trillion C-14. As long as we eat and breathe, our carbon is one trillionth C-14.When we die, the C-14 decays (with its 6 thousand year half-life) but it is no longer replaced. After you are buried for 6 kyr, the amount of C-14 in your body is reduced by half. In another 6 kyr, it is cut in half again. By measuring the ratio of C-14 to ordinary carbon, we know when you died (or the tree, or the fossil, or whatever).All potassium contains a 0.01% (i.e. one part in 10,000) of the radioactive isotope K-40. [Ordinary potassium is K-39] The half life of K-40 is 1.26 billion years. When it decays, the K-40 emits an electron, a neutrino, and the remaining nucleus turns into Argon-40, abbreviated Ar-40.This is particularly useful for geology. Imagine that there was a volcanic eruption in the past, and a molten rock lands in the sea. Because the rock was hot and molten, any gas that was trapped inside it escapes. When the rock solidifies, it initally has no gas trapped inside. But here is potassium inside, and therefore there is also potassium-40 inside. Every years, some of that K-40 decays, turning into argon gas. Because the rock is solid, the gas cannot escape.This rock might be trapped in sea floor sediment, where it becomes part of newly forming sedimentary rock. A million years later, this sedimentary rock might have been lifted above the sea as mountains formed. We examine the rock, and we see an interesting fossil. Perhaps the fossil of a dinosaur. We wonder, how old is the fossil? In the same sedimentary rock, the astute geologist notices a rock that he can tell (he is a geologist!) came from a volcano. He brings the rock to the laboratory, melts it, and measures the amount of argon gas that comes out. From that measurement, the geologist knows how long ago the rock was formed, and so knows when the dinosaur bone was laid down.In nuclear science, the decay chain refers to the radioactive decay of different discrete radioactive decay products as a chained series of transformations. Most radioactive elements do not decay directly to a stable state, but rather undergo a series of decays until eventually a stable isotope is reached.

Decay stages are referred to by their relationship to previous or subsequent stages. A parent isotope is one that undergoes decay to form a daughter isotope. The daughter isotope may be stable or it may decay to form a daughter isotope of its own. The daughter of a daughter isotope is sometimes called a granddaughter isotope.IThe time it takes for a single parent atom to decay to an atom of its daughter isotope can vary widely, not only for different parent-daughter chains, but also for identical pairings of parent and daughter isotopes. While the decay of a single atom occurs spontaneously, the decay of an initial population of identical atoms over time, t, follows a decaying exponential distribution, e-λt, where λ is called a decay constant. Because of this exponential nature, one of the properties of an isotope is its half-life, the time by which half of an initial number of identical parent radioisotopes have decayed to their daughters. Half-lives have been determined in laboratories for thousands of radioisotopes (or, radionuclides). These can range from nearly instantaneous to as much as 1019 years or more.nNuclear PhysicsRadiation & Half LifeJJ ThomsonAtomic ModelsJ DaltonRutherfordBohrSir Joseph John “J. J.” Thomson, OM, FRS (18 December 1856 – 30 August 1940) was a British physicist and Nobel laureate, credited for the discovery of the electron and of isotopes, and the invention of the mass spectrometer. He was awarded the 1906 Nobel Prize in Physics for the discovery of the electron and his work on the conduction of electricity in gases.Niels Henrik David Bohr (7 October 1885 – 18 November 1962) was a Danish physicist who made fundamental contributions to understanding atomic structure and quantum mechanics, for which he received the Nobel Prize in Physics in 1922. Bohr mentored and collaborated with many of the top physicists of the century at his institute in Copenhagen. He was also part of the team of physicists working on the Manhattan Project. Bohr married Margrethe Nørlund in 1912, and one of their sons, Aage Niels Bohr, grew up to be an important physicist who in 1975 also received the Nobel prize. Bohr has been described as one of the most influential physicists of the 20th century.John Dalton FRS (6 September 1766 – 27 July 1844) was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness (sometimes referred to as Daltonism, in his honour).Five main points of Dalton's Atomic Theory* Elements are made of tiny particles called atoms. * All atoms of a given element are identical. * The atoms of a given element are different from those of any other element; the atoms of different elements can be distinguished from one another by their respective relative weights. * Atoms of one element can combine with atoms of other elements to form chemical compounds; a given compound always has the same relative numbers of types of atoms. * Atoms cannot be created, divided into smaller particles, nor destroyed in the chemical process; a chemical reaction simply changes the way atoms are grouped together.Hiplum pudding modelProposed in 1904 before the discovery of the atomic nucleus. In this model, the atom is composed of electrons (which Thomson still called "corpuscles", though G. J. Stoney had proposed that atoms of electricity be called electrons in 1894) [1] , surrounded by a soup of positive charge to balance the electron's negative charge, like negatively-charged "plums" surrounded by positively-charged "pudding". The electrons (as we know them today) were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons. Instead of a soup, the atom was also sometimes said to have had a cloud of positive charge.ElectronProtonNeutronDiscovered:DetailsIn atomic nucleusNeutral charge1.00866uJames Chadwick1932DiscoveredDiscovered1897JJ Thomson1919E. RutherfordDetailsNegative ChargeFound around nucleus in an electron 'Cloud'mass - 5.485799×10 uAtomic NumberMass NumberIsotopesRutherford's ExperimentZIn chemistry and physics, the atomic number (also known as the proton number) is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element. In an atom of neutral charge, atomic number is equal to the number of electrons.The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. The mass number is different for each different isotope of a chemical element.AIsotopes (Greek isos = "equal", tópos = "site, place") are any of the different types of atoms (nuclides) of the same chemical element, each having a different atomic mass (mass number). Isotopes of an element have nuclei with the same number of protons (the same atomic number) but different numbers of neutrons. Therefore, isotopes of the same element have different mass numbers (number of nucleons).CarbonC14C12HydrogenHHH132HydrogenDeuteriumTritiumUraniumUU235238ChlorineCl3517ArgonAr4022IronFe5626BoronB95Planetary ModelRutherford's new model for the atom, based on the experimental results, had a number of essential modern features, including a relatively high central charge concentrated into a very small volume in comparison to the rest of the atom and containing the bulk of the atomic mass (the nucleus of the atom), and a number of tiny electrons circling around the nucleus like planets around the sun.DetailsIn atomic nucleusPositive tcharge1.00728uNuclear Physics-4Atomic StructureErnest Rutherford, 1st Baron Rutherford of Nelson, OM, FRS (30 August 1871 – 19 October 1937) was a New Zealand born British chemist and Physicist who became known as the father of nuclear physics. He discovered that atoms have a small charged nucleus, and thereby pioneered the Rutherford model (or planetary model, which later evolved into the Bohr model or orbital model) of the atom, through his discovery of Rutherford scattering with his gold foil experiment. He was awarded the Nobel Prize in Chemistry in 1908. He is widely credited as splitting the atom in 1917 and leading the first experiment to "split the nucleus" in a controlled manner by two students under his direction, John Cockcroft and Ernest Walton in 1932.ISOTOPESExperimental ResultsAlmost all the alpha particles went straight through (did NOT deviate from their original path)A few were deflected by a small amount.A very few were deflected by large angles – even up to 180 degrees.ImplicationsThe positive nucleus contains almost all the mass of the atom and is much more massive than the alpha particles.There is a nucleus which is very small and positively charged.The gold atom was almost all empty space.ExperimentsCathode RaysCathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube.To release electrons into the tube, they first must be detached from the atoms of the cathode.In the early cold cathode vacuum tubes, called Crookes tubes, this was done by using a high electrical potential , modern vacuum tubes use thermionic emission.Since the electrons have a negative charge, they are repelled by the cathode and attracted to the anode. They travel in straight lines through the empty tube. The voltage applied between the electrodes accelerates these low mass particles to high velocities. Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall of the tube, exciting the atoms of the glass and causing them to emit light, a glow called fluorescence.1st Experiment2nd Experiment3rd ExperimentThomson's ExperimentsIn a variation of an 1895 experiment by Jean Perrin, Thomson built a cathode ray tube ending in a pair of metal cylinders with a slit in them. These cylinders were in turn connected to an electrometer, a device for catching and measuring electrical charge.Perrin had found that cathode rays deposited an electric charge. Thomson wanted to see if, by bending the rays with a magnet, he could separate the charge from the rays.He found that when the rays entered the slit in the cylinders, the electrometer measured a large amount of negative charge. The electrometer did not register much electric charge if the rays were bent so they would not enter the slit. As Thomson saw it, the negative charge and the cathode rays must somehow be stuck together: you cannot separate the charge from the rays.All attempts had failed when physicists tried to bend cathode rays with an electric field. Now Thomson thought of a new approach. A charged particle will normally curve as it moves through an electric field, but not if it is surrounded by a conductor (a sheath of copper, for example).Thomson suspected that the traces of gas remaining in the tube were being turned into an electrical conductor by the cathode rays themselves. To test this idea, he took great pains to extract nearly all of the gas from a tube, and found that now the cathode rays did bend in an electric field after all."What are these particles?Are they atoms, or molecules, or matter in a still finer state of subdivision?"

Thomson concluded from these two experiments,

"I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter." But, he continued, "What are these particles? are they atoms, or molecules, or matter in a still finer state of subdivision?"This sought to determine the basic properties of the particles. Although he couldn't measure directly the mass or the electric charge of such a particle, he could measure how much the rays were bent by a magnetic field, and how much energy they carried. From this data he could calculate the ratio of the mass of a particle to its electric charge (m/e). He collected data using a variety of tubes and using different gases.The results were astounding. Just as Emil Wiechert had reported earlier that year, the mass-to-charge ratio for cathode rays turned out to be far smaller than that of a charged hydrogen atom--more than one thousand times smaller. Either the cathode rays carried an enormous charge (as compared with a charged atom), or else they were amazingly light relative to their charge.Thomson boldly announced the hypothesis that

"we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter... is of one and the same kind; this matter being the substance from which all the chemical elements are built up."ExperimentsCathode RaysCathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube.To release electrons into the tube, they first must be detached from the atoms of the cathode.In the early cold cathode vacuum tubes, called Crookes tubes, this was done by using a high electrical potential , modern vacuum tubes use thermionic emission.Since the electrons have a negative charge, they are repelled by the cathode and attracted to the anode. They travel in straight lines through the empty tube. The voltage applied between the electrodes accelerates these low mass particles to high velocities. Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall of the tube, exciting the atoms of the glass and causing them to emit light, a glow called fluorescence.1st Experiment2nd Experiment3rd ExperimentThomson's ExperimentsIn a variation of an 1895 experiment by Jean Perrin, Thomson built a cathode ray tube ending in a pair of metal cylinders with a slit in them. These cylinders were in turn connected to an electrometer, a device for catching and measuring electrical charge.Perrin had found that cathode rays deposited an electric charge. Thomson wanted to see if, by bending the rays with a magnet, he could separate the charge from the rays.He found that when the rays entered the slit in the cylinders, the electrometer measured a large amount of negative charge. The electrometer did not register much electric charge if the rays were bent so they would not enter the slit. As Thomson saw it, the negative charge and the cathode rays must somehow be stuck together: you cannot separate the charge from the rays.All attempts had failed when physicists tried to bend cathode rays with an electric field. Now Thomson thought of a new approach. A charged particle will normally curve as it moves through an electric field, but not if it is surrounded by a conductor (a sheath of copper, for example).Thomson suspected that the traces of gas remaining in the tube were being turned into an electrical conductor by the cathode rays themselves. To test this idea, he took great pains to extract nearly all of the gas from a tube, and found that now the cathode rays did bend in an electric field after all."What are these particles?Are they atoms, or molecules, or matter in a still finer state of subdivision?"

Thomson concluded from these two experiments,

"I can see no escape from the conclusion that [cathode rays] are charges of negative electricity carried by particles of matter." But, he continued, "What are these particles? are they atoms, or molecules, or matter in a still finer state of subdivision?"This sought to determine the basic properties of the particles. Although he couldn't measure directly the mass or the electric charge of such a particle, he could measure how much the rays were bent by a magnetic field, and how much energy they carried. From this data he could calculate the ratio of the mass of a particle to its electric charge (m/e). He collected data using a variety of tubes and using different gases.The results were astounding. Just as Emil Wiechert had reported earlier that year, the mass-to-charge ratio for cathode rays turned out to be far smaller than that of a charged hydrogen atom--more than one thousand times smaller. Either the cathode rays carried an enormous charge (as compared with a charged atom), or else they were amazingly light relative to their charge.Thomson boldly announced the hypothesis that

"we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter... is of one and the same kind; this matter being the substance from which all the chemical elements are built up."